The present invention relates to a photomask used to transfer a pattern onto a wafer by exposure in a photolithography step of a semiconductor manufacturing process and a photomask blank for forming the photomask and, more particularly, to a halftone phase shift photomask or a blank for the photomask and methods of manufacturing these.
This application is based on Japanese Patent Application No. 8-282409, filed Oct. 24, 1996 and Japanese Patent Application No. 8-322521, filed Dec. 3, 1996, the content of which is incorporated herein by reference.
Conventional photomasks have the following problem. If fine patterns are projected onto a wafer to expose the wafer, and the patterns (light transmitting portions) are close to each other, lights transmitted through the light transmitting portions are diffracted and interfere with each other to increase the light intensity in the boundaries of the patterns (i.e., light-shielding (or opaque) portions). Consequently, the photoresist is exposed to light in light-shielding portions, and this makes the patterns transferred onto the wafer impossible to separate and resolve. This phenomenon becomes more perceptible as the pattern size closes to the wavelength of the exposure light. In principle, it has been impossible to separate and resolve fine patterns with geometries smaller than the wavelength of the exposure light by using conventional photomasks and conventional exposure optical systems.
Accordingly, a phase shift mask using a phase shift technology has been developed. The phase shift mask utilizes a difference in transmitting speed of light transmitted through various materials so that transparent thin films (phase shifters) are partially formed on the mask. The phase shift between the light transmitted through the phase shifter and the light not transmitted through the shifter improves the resolution.
The phase shift mask includes a Levenson type and a halftone type. Examples of the Levenson type phase shift mask are disclosed in Japanese patent KOKAI publication No. 58-173744 and Japanese patent KOKOKU publication No. 62-50811 (in which only the principle is described).
In the Levenson type phase shift mask, the lights transmitted through adjacent light transmitting portions have a phase shift of 180.degree.. One of the adjacent light transmitting portions has a phase shifting function. When the transmitted lights are diffracted and interfere with each other, the light intensity in the boundaries of the patterns is decreased so that the fine pattern transferred onto the wafer can be separated and resolved. This relationship holds both before and after the focal point. Therefore, even if the focal point somewhat gets out of position, the resolution is improved than in conventional methods, and this improves the focusing margin.
However, these Levenson masks have not been put into practical use yet because their pattern design and manufacturing processes are complicated.
Examples of a halftone type phase shift mask are described in Japanese patent KOKAI publication No. 4-136854 and U.S. Pat. No. 4,890,309.
In the halftone masks, a light-shielding layer (non-transmitting layer) for forming patterns is not a complete light-shielding layer but a halftone layer which transmits a slight amount (a few %) of light. The optical constants of the halftone layer are such determined that the light passing through the halftone and the light not passing through the halftone have a phase shift of 180.degree.. Such halftone phase shift masks are particularly effective in improving the resolution of an isolated pattern.
In addition to a phase shift amount of 180.degree. as the original optical constant of a phase shift mask, a halftone phase shift mask and a mask blank must simultaneously have a low transmittance (2 to 15%) and a low reflectance (30% or less) at the exposure light wavelength, a low transmittance (30% or less) at the inspection light wavelength, and conductivity to some extent (100 M .OMEGA./.quadrature. or less).
The reasons are as follows. If the reflectance of a photomask at the exposure light wavelength is high (more than 30%), multiple reflection occurs between the halftone layer and the wafer when photolithography is performed. This decreases the pattern accuracy.
Also, inspection and size measurement of a phase shift mask use light in the visible light region (e.g., the e-line (wavelength: 546 nm) of a high-pressure mercury lamp, an Ar ion laser beam (wavelength: 488 nm), or an He--Ne laser beam (wavelength: 633 nm)). If the transmittance is high (more than 30%) at the inspection light wavelength, the contrast between apertures forming patterns and the halftone layer decreases. This makes the inspection and size measurement difficult to perform.
Additionally, in patterning the halftone layer, if the conductivity of the halftone layer is low in electron-beam lithography for exposing an electron-beam resist, electrons are charged up to make accurate patterns impossible to form and allow easy adhesion of dust in manufacturing steps of a mask or when the mask is in use due to charging of static electricity.
To avoid the above problems, some phase shift masks are formed by overlapping two or more halftone films in order to decrease the reflectance to exposure light or give conductivity to the film surface. Examples are masks using molybdenum silicide and chromium compound. However, a number of problems have been pointed out for these films; the transmittance is high in the inspection light wavelength, the controllability and the reproducibility of these films are lower in a manufacturing step as the exposure light wavelength is shortened.